@Rubén it is a drop-down (through content validation) of three possible options ("started", "ongoing", "finished")
– luisgonzalezJan 19 at 18:05

@Rubén, for what understand the referred code works showing the time it was updated last i.e. every time the target cell is modified, the destination is also updated to the most recent time. I do not want this. I want the cell to store the first time the target cell was modified, and never change again.
– luisgonzalezJan 19 at 19:38

Me voy a registrar en SO en español! In the meantime since I already asked here we can continue here. I tweaked the code in that sample (removing static references from column or sheets) but I think something breaks in the middle because the script then enters an infinite loop and keeps adding timestamps (even if the function is removed! from a cell). Maybe there's a better way to make that referred code more generalistic and flexible so applies to any target or destination cell?
– luisgonzalezJan 19 at 20:50